The manufacture of semiconductor devices typically involves applying a layer of a photosensitive substance (a photoresist) to the surface of a target wafer. The photoresist is exposed to light in a selected pattern using a photomask, and the photoresist is then developed to leave exposed regions of the wafer. Typically, the exposed regions are subsequently etched away or otherwise modified, and the residual photoresist is removed. The pattern of the photomask typically possesses extremely fine details, and the presence of even tiny particles on the surface of the photomask can interfere with the accurate reproduction of the pattern on the target wafer.
To minimize particulate contamination at the mask surface, optical pellicles have been developed that protect the photomask. An optical pellicle is a frame-mounted transparent membrane that is attached to the photomask surface, so that contaminating particles fall onto the pellicle membrane and not the surface of the photomask. The pellicle frame holds the pellicle membrane at a sufficient distance above the mask surface so that any particles that may fall upon the membrane lie outside the focal plane of the illuminating light, and so fail to interfere with the projected mask pattern. The use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and has become widespread in the industry.
Pellicles may be packaged such that a protective cover is applied to the pellicle frame, opposite the side of the pellicle. This may create an airtight package with the pellicle, frame, and protective cover. Alternatively, a pellicle may be shipped already adjacent to the photomask, also forming an airtight package.
Pellicle packages are typically airtight to reduce contamination from dust and other contaminate particles. A problem occurs when temperature and pressure change during shipping. This may cause the packaging and/or pellicle to expand and/or contract and pellicles may be damaged as a result. Therefore, pressure equalization orifices have been designed in the frame and in the adhesive layer of a pellicle configuration, some with filters, to allow for pressure equalization without damaging the pellicle and without allowing significant amounts of contaminate particles to enter the area between the pellicle and photomask. However, constructing these orifices is costly and time consuming, and filters are sometimes not effective in filtering out smaller dust and other small particulate.
A related problem has arisen when pellicles are shipped after they have been mounted to a photomask. In this instance, a filtered orifice may not be usable because minute particles enter the orifice and pass through the filter into the space between the pellicle and the photomask. These particles tend to become deposited directly on the photomask, this defeating the purpose of the pellicle.
It is an object of the present invention to overcome the drawbacks of the prior art proposals. Specifically, it is an object of the present invention to develop a system for equalizing the pressure when a pellicle-protected photomask is shipped by air or is otherwise exposed to conditions that cause ambient pressure to increase or decrease.